Field of the Invention
The present invention relates generally to transmitters, and more particularly to a self-oscillating transmitter which uses feedback to maintain the oscillation at the resonant frequency of the load, thereby optimizing the efficiency of operation and overcoming the frequency mismatching inherent in previously known transmitters.
Of the various types of power amplifiers in common use, Class D power amplifiers offer a particularly high degree of efficiency, and are thus quite desirable in applications in which efficiency is an important feature. Such applications include battery powered devices in which power conservation is critical to an extended operational life without battery recharging or replacement. One example of such an application is in programming devices used to program cardiac pacemakers.
Such programming devices, or programmers, operate on batteries, and use a telemetry head which contains a coil to communicate with an implanted pacemaker having a coil therein. By using magnetic coupling between the external coil in the telemetry head and the internal coil in the pacemaker, information may be transmitted between the programmer and the pacemaker. Such information transmitted may include, for example, device identification information, biological data, current operational parameters of the device, technical information regarding proper operation of the device, battery charge condition, revised operational parameters (programming information) for the device, and verification of information transmitted between the implanted device and the external transceiver.
It will be appreciated by those skilled in the art that it is desirable to extend the operating life of batteries in the programmer as much as is possible. Accordingly, a Class D transmitter is a highly desirable type of amplifier for use in a programmer. A Class D transmitter is typically implemented using a pair of switches connected in series with a battery. By driving the switches with a driver at a selected frequency f.sub.0 to alternately close the switches, and by taking a tap at the point between the two switches, a square wave may be obtained. The driving frequency f.sub.1 is selected because it is the resonant frequency of the LC combination in the transmitter.
This square wave is supplied to one side of a capacitor, the other side of which is connected to one side of an inductor which is the coil used to transmit the information. The other side of the inductor is grounded, and thus the voltage across the inductor will be a sinusoidal wave. By operating the driver to produce the square wave, which is converted into the sine wave, a digital "one" is transmitted. Similarly, by not operating the driver so that no square wave will be generated, the sine wave will quickly decay, whereby a digital "zero" is transmitted.
This Class D power amplifier is highly efficient, since transistors are used as the switches. In theory, Class D amplifiers are 100 percent efficient, although in practice it requires some power to operate the transistor switches. Thus, Class D power amplifiers are ideal for use in the application mentioned above, as the transmitter used in a pacemaker programmer.
Unfortunately, while this situation is ideal in theory, in practice there is at least one significant problem encountered in operating a Class D transmitter of the design described above in a pacemaker programmer. When the coil in the telemetry head of the programmer is brought into proximity to the coil in the programmer, the inductance of each coil will be affected by the other coil. As the coils get closer, the inductance of the coil in the telemetry head will be reduced by the proximity of the coil in the pacemaker.
The effect of this reduction in the inductance of the coil in the telemetry head is that the resonant frequency of the LC combination will also be changed. As the inductance is reduced by the proximity of the coil in the pacemaker, the resonant frequency of the capacitor together with the inductor in the telemetry head will increase to a new frequency f.sub.1, which is higher than the original resonant frequency f.sub.0. Thus, the driver will attempt to forcibly drive the transmitter at a frequency which is not the true resonant frequency of the LC combination in the transmitter. This results in a smaller voltage across the inductor, which reduces the signal and the efficiency of the transmitted signal.
It is accordingly the primary objective of the present invention that it provide a Class D transmitter which is not susceptible to the frequency mismatch problem of existing Class D transmitters. The efficiency of the transmitter should be maximized by avoiding the frequency mismatch which is characteristic in the art, thereby preventing losses by driving the LC combination in the transmitter at a non-resonant frequency. Thus, in operation the frequency of the square wave oscillation must be identical to the resonant frequency of the LC combination in the transmitter.
It is thus a secondary objective of the present invention that it control the frequency that the square wave signal is generated at. In this manner, the operational frequency is controlled precisely at the resonant frequency of the LC combination in the transmitter. While meeting all of the above objectives and guidelines, it is essential that the transmitter of the present invention continue to operate as a Class D transmitter to maximize the efficiency thereof. It is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.